Dielectric layer polarizers of the polarizing cube type have been well known in optics for about 40 years.
They were invented by MacNeille and manufactured by Banning. They direct the two s and p components of the incident light beam along two perpendicular directions of propagation. These components are such that the direction of the vibration of the electric field are respectively perpendicular and parallel to the plane of incidence of the beam. Such polarizers make use of the fact that reflection and transmission of light through a stack of thin layers deposited on a support through which the incident light passes at an oblique angle of incidence are different depending on the direction of vibration of the light.
Such a polarizer is made in the form of a cube which is generally made of glass and which is cut along a plane passing through two opposite edges in order to reveal two facing oblique faces referred to as hypothenuse faces. At least one of these faces is coated with a polarizing stack. The angle of each of the two prisms constituted by the cube is 45.degree., and as a result the prisms must be glued together by their hypothenuse faces in order to avoid total internal reflection of the light on one of these faces inside the cube. The inlet and outlet faces of the cube are generally given antireflection treatment.
The polarizing stack is generally constituted by alternating layers of low and high index n.sub.H and n.sub.N. The literature (see documents 1 to 4) specifies the conditions that must be satisfied by the refractive indices of the deposited layers and the refractive index n.sub.O of the substrate constituted by the prism(s) carrying the layers in order to ensure that the optical thickness of the layers is indeed quarterwave oblique incidence.
The thickness of a layer is said to be quarterwave for light of a given wavelength received at a given angle of incidence when the two fractions of the light reflected by the two faces of the layer are antiphase. A layer of half that thickness is said to be one eighth of a wavelength thick.
In order to be effective, it is necessary for the transmittance T.sub.p of the p vibration through a polarizer to be close to unity while its transmittance T.sub.s of the s vibration is practically zero. Nowadays, a good polarizer should have transmittance T.sub.p of at least 0.97 and transmittance T.sub.s of less than 10.sup.-2, thereby providing relative attenuation of the s vibration of at least 20 dB.
Multidielectric layer polarizers are known which are effective over a wide range of the spectrum. Unfortunately, they are effective only over a small range of angles, typically less that .+-.2.degree. about the normal incidence to the front face of the polarizing cube. More precisely, an insertion loss of more than 0.5 dB is observed for the p vibration once the angle of incidence on the front face of the polarizer exceeds 2.degree.. Such loss is excessive.
A particular object of the present invention is to provide a dielectric layer polarizer which avoids excessive transmission loss of the p vibration when the angle of incidence of the beam to be polarized varies over more than 2.degree. while the spectrum width of the beam is limited. Another object of the invention is to provide polarizers which are effective over a wide range of angles, about .+-.10.degree., or light which is substantially monochromatic. Such polarizers may be referred to as wide angle polarizers.
Possible applications of wide angle polarizers include polarizing a beam of infrared light from a semiconductor lazer emitting at a wavelength of 1.3 microns (.mu.m) or 1.55 .mu.m. Such a beam has the advantage of being monochromatic. However it also suffers from the drawback of having a divergence of several degrees. If a convention MacNeille type polarizer is used, it is necessary to add a lens system for collimating the laser beam prior to passing it through the polarizer. The use of wide angle polarizer means that the collimator can be omitted, thereby giving an assembly which is cheaper, more compact, and which minimizes insertion losses.